A. Field of the Invention
The present invention relates to signal synchronization in multi-cell communication systems. The disclosed method and apparatus are applicable to all communication systems requiring frame synchronization, especially those using synchronized interfaces. These systems include, but are not limited to, Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) systems. The present invention is intended for use in, for example, wireless communications, such as cordless telephone systems; low-tier, low-power, multi-cell personal communications services; wireless local loops (WLL); wireless local area networks (LAN); and wireless private branch exchanges (PBX).
B. Description of the Related Art
FIG. 1 shows a typical configuration of a conventional mobile communication system. A central control fixed part (CCFP) 10 is typically connected to several radio frequency parts (RFP) 20, 30, and 40, via two-way communication links 120. The CCFP may also be connected to a control terminal 50. In the particular configuration shown in FIG. 1, the CCFP is connected to another processor 60, which is connected to a telephone network 80. It is possible to connect, for example, communication devices 70 directly to processor 60 as shown.
In conventional usage, the CCFP 10 and RFPs 20, 30, and 40 may also be referred to as a master station and slave base stations, respectively. The RFPs typically have an effective signal transmission radius 90, which defines their respective areas of coverage, or cells, 100, shown in FIG. 1 as dashed circles. Within these cells 100 are located mobile communication devices 110, which are capable of two-way or one-way wireless communication with the RFP in whose area they reside. These mobile communication devices 110 include, but are not limited to, cordless telephones, cellular telephones, pagers, personal digital assistants (PDA), and mobile computers.
In a mobile communication system such as shown in FIG. 1, the CCFP 10 communicates with RFPs 20, 30, and 40 via an Integrated Services Digital Network (ISDN) communication protocol. RFPs 20, 30, and 40 typically communicate with devices 110 via a Digital European Cordless Telecommunications (DECT) communication protocol. Both of these communication protocols are known to those skilled in the telecommunications art, and will not be further described here. Both the ISDN and DECT protocols transmit data in structured frames. It is desirable for a given frame of data transmitted by RFPs 20, 30, and 40 to be transmitted at the same time by all RFPs; this is known as frame synchronization.
Frame synchronization (FS) in a multi-cell system is a necessary step to reduce interference between users and signal acquisition time. Unfortunately, because the RFPs are different distances from the CCFP, even if the CCFP transmits a synchronization signal, the signal will arrive at the RFPs at different times due to different propagation delays from the CCFP to the respective RFPs. Based on an acceptable interference criterion, it has been shown that the maximum timing error allowed for FS is 11 microseconds for the DECT protocol (e.g., in Lu, C. C., “Frame synchronization and capacity of DECT Systems”, CCL Technical Journal, 1996). If propagation delays are longer than this time, additional steps are needed for FS besides a transmitted synchronization signal.
Conventionally, the principal steps needed to implement FS are measuring the unique signal time delay between master and slave bases, and adjusting frame clocks at the slave bases so that the transmission times from all base stations are the same. This latter subject has been treated in the prior art and there are many methods of implementation. One approach taken by a CDMA system with multiple bases is using a synchronized clock derived from a global positioning system (GPS) signal. Another conventional approach is to update the FS when the system load is low, e.g., at midnight.
Another approach for FS involves additional wiring, shown as wiring 130 in FIG. 1, which is dedicated for frame synchronization only. Such synchronization may be implemented by an independent star network, and this method performs satisfactorily. Using this system, the time delays for each base station may be compensated for with high accuracy, but the required extra wiring 130 for FS only is wasteful.